In these early days, I think we'll probably be more focused on targets of opportunity, but once the technology base exists to be picky, if you could locate a body full of precious metals, say gold and platinum, it could easily dwarf the earthbound supply.
I think the most interesting opportunities lie not with traditional methods of exploiting these materials, but with the methods we will discover once previously rare materials become abundant and cheap.
Of course, devaluing gold to the point of aluminum is going to cause political upheaval and that will have to be dealt with, and while this conjures images of chaos and serious trouble, it doesn't seem like all that big of a deal compared to what will happen when a private entity starts talking about bringing a dinosaur-killer-sized body near the earth.